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LIME

LIME

This is the product of the calcination of limestone and, when mixed with
siliceous sand, forms the well-known common mortar. Limes are distin-
guished as fat limes, consisting essentially of calcium oxide and readily
slaked with evolution of considerable heat and with increase of volume,
and poor limes, containing marked quantities of clay, magnesia and other
impurities.

The chemical analysis of lime is analogous to that of limestone (see
Limestones and Marls), but usually the determination of carbon dioxide
is unnecessary. For industrial purposes, however, certain technical tests
generally suffice, these being principally:

1.  Ease of Storage.—This is deduced from the greater or less time
required for a lime exposed to the air to become slaked and fall into powder
in contact with the moisture of the air.

2.  Ease of Slaking.—This is ascertained by observing if the lime,
mixed with 3-4 times its weight of water, undergoes hydration in a short
time with pronounced generation of heat and formation of a dense paste.

3.  Volume  after   Slaking.—This is  determined by means of the
Michaelis volumenometer, consisting of a brass vessel into the orifice of
which a glass tube with graduations starting from 200 c.c. may be screwed
air-tight.    In the container a piece of the lime weighing 25-50 grams is
slaked with the amount of water necessary to obtain a dense paste, heat
being applied, where necessary, by a water-bath and also occasional stirring
until cracks begin to develop in the paste.    The latter is then allowed to
cool,  the graduated tube fitted, 200 c.c.  of water   introduced from a
pipette, and the volume read.

Another method, based on the use of a porous cylinder and capable of
giving good results, has been described by G. Giorgis and G. Cenni.1

In practice, owing to the difficulty of obtaining a small sample repre-
senting the bulk of a lump lime, a large quantity of the substance (at least
5 kilos) is sprinkled with water, left in the air for at least two days, after
which water is added to give a paste which can be poured, this being then
introduced (through a sieve to retain inert matter) into a bucket of known
dimensions ; after the excess of water has been separated from the surface
of the paste, the level of the latter is read off and its volume calculated.

*
* *

Fat lime for the prochiction of mortar should be of recent and thorough kiln-
ing, and not powdery or effloresced ; it usually contains not less than 95% CaO,
mostly free. It should be non-vitreous and of uniform colour, and when mixed
with the necessary quantity of water it should undergo rapid slaking with trans-
formation into a firm paste, without leaving any appreciable residue due to insuf-
ficiently burnt, siliceous or otherwise inert matter ; the yield is usually not less
than 2-5 cub. dm. of paste per kilo of quicklime.

1 Annali di Chim, applicata, 1915, III, p. 175.7